1. Field of the Invention
The present invention relates to a power supply device that produces from an input voltage a predetermined output voltage to be supplied to a load in such a way that a monitor voltage that varies according to the output voltage is kept equal to a reference voltage. The present invention relates also to an electric appliance employing such a power supply device.
2. Description of the Prior Art
In a power supply device that produces from an input voltage a predetermined output voltage to be supplied to a load in such a way that a monitor voltage that varies according to the output voltage is kept equal to a reference voltage, feedback control is performed so that, even when the input voltage or the current flowing through the load (hereinafter referred to as the load current) varies to a certain degree, the output voltage is kept at a predetermined level.
However, even though feedback control as described above is performed, the output voltage of a power supply device generally tends to increase as the input voltage increases and decrease as the load current increases. Thus, a power supply device configured as described above has the disadvantage of being unable to keep the output voltage at a predetermined level when the input voltage or the load current varies greatly.
To overcome this, in a conventional power supply device, variation in the output voltage resulting from variation in the power source or the load is reduced by adjusting, according to the load current, the gain of a regulator IC used as an output control means (refer to, for example, Japanese Patent Application Laid-Open No. 2000-47738).
It is true that, in a power supply device configured as described just above, variation in the output voltage resulting from variation in the power source or the load can be reduced so that the output voltage is kept within a predetermined range (within the rated range of voltages in which the load is supposed to operate).
However, in an attempt to solve the above problem through the adjustment of the gain of a regulator IC, it is extremely difficult to strike a proper balance between the reduction of variation in the output voltage resulting from variation in the power source or the load (i.e., the increasing of the gain of the regulator IC) and the likeliness of oscillation, because there is a tradeoff between these two factors.
In particular, lately, as increasingly strict requirements are imposed on the specifications of the power to be supplied to a load, increasingly precise power supply devices are sought that are less prone to variation in the output voltage even in the face of great variation in the input or the load. To realize such a power supply device, it is necessary to vary the gain widely according to the input voltage or the load current so as to minimize variation in the output voltage, and simultaneously secure a sufficient phase margin to avoid oscillation over the entire range of the gain. This makes the designing of a power supply device extremely difficult.